CONSTRAINTS ON PROCESSES AFFECTING THE ORIGIN OF OCEANIC CRUST: GEOCHEMICAL EVIDENCE FROM THE 0-35 M. Y. AGE BASALTS, BETWEEN 30°N AND 40°N, MAR

Abstract

DSDP Leg 82 sampled the oceanic crust, at 9 sites, west of the MAR between 33°N and 38°N, i.e., from the Azores “mantle plume” to south of the Hayes Fracture Zone. A specific aim of this DSDP Leg was to determine if the origin, and spatial and temporal evolution of oceanic mantle heterogeneity in the vicinity was related in a predictable manner to the “mantle plume”. Our approach in evaluating this question has been to select samples on the basis of trace element characteristics and age, for radiogenic isotope determinations.Isotopic and trace element heterogeneity occur both within between sites. While there is a crude correlation between trace element and isotopic characteristics, it is difficult to establish a detailed set of guidelines which would describe either trace element-isotope interrelationships. This difficulty is highlighted by the occurrence of basalts in one hole (H558) with trace element characteristics ranging from enriched to depleted, which can have identical or different isotopic characteristics.The origin, and spatial and temporal distribution of the heterogeneity cannot be reconciled with any simple models. A complicated scenario, involving mantle plums or blobs, recent local heterogeneity and complex mixing models, is required.